About & methodology

BTU Calculator is a free tool that estimates how much cooling (and heating) a space needs, in BTU and tons, from its size and conditions. No sign-up, no email wall — the numbers are computed in your browser.

How cooling is estimated

We start from the ENERGY STAR baseline of roughly 20 BTU per square foot (assuming an 8-ft ceiling) and apply documented adjustments:

• Ceiling height scales the load (height ÷ 8 ft).
• Insulation: poor +15%, good −10%.
• Sun exposure: very sunny +10%, heavily shaded −10% (ENERGY STAR).
• Climate zone: hot +25% down to cold −15% for cooling.
• Occupants: +600 BTU per person beyond two (ENERGY STAR).
• Kitchen: +4,000 BTU (ENERGY STAR).

The total is rounded up to the next standard AC size, and tonnage is BTU ÷ 12,000.

How heating is estimated

Furnace output is sized by climate zone using published BTU/sq-ft factors, from about 32 BTU/sq-ft in the hottest zones up to 55 BTU/sq-ft in the coldest, adjusted for ceiling height and insulation.

Important

These are planning estimates from a simplified model, not a substitute for a professional Manual J load calculation. Confirm with a licensed HVAC contractor before buying or installing equipment.